Chain “vibrates” on higher gearsIt's not the case, the folding mechanism is fine and holds the frame in place firmly. The bottom bracket, is a bit loose so it wobbles a little bit when pedalling but as I can judge that doesn't change chain's angle that much.

Chain “vibrates” on higher gearsLast year I have travelled roughly 5300 km on my bike (I know because almost every ride is recorded with GPS). That makes sense then, it was too much for my poor chain. Thank you!